Position sensing tools have several important applications in downhole tools used in subterranean drilling. For example, many drilling applications require directional drilling tools to control the lateral drilling direction. Such steering tools commonly include a plurality of force application members (also referred to herein as blades) that may be independently extended out from and retracted into a housing. The blades are disposed to extend outward from the housing into contact with the borehole wall and to thereby displace the housing from the centerline of a borehole during drilling. Blade position sensors are useful for determining blade extension. Accurate blade position measurements facilitate more accurate steering of the drill bit. Additionally, such blades are typically controlled by a hydraulic circuit. The measurement of a piston position within a hydraulic reservoir may be utilized, for example, to calculate the volume of pressurized hydraulic fluid available to actuate the blades.
Various position and displacement sensors are known in the downhole arts for measuring the position of pistons, blades, and other movable components on downhole tools (e.g., including wireline tools, logging-while-drilling tools, measurement-while-drilling tools, and steering tools). Such sensors typically make use of analog sensing devices such as potentiometers, pressure transducers, or ultrasonic transducers. For example, Webster, in U.S. Pat. No. 5,603,386 discloses a downhole steering tool in which each blade is fitted with a sensor (such as a potentiometer) for measuring the borehole size and the displacement of the blade.
While prior art sensors are known to be serviceable, such as for measuring blade and/or piston position, they are also known to suffer from various drawbacks. For example, potentiometers are known to be susceptible to mechanical wear and temperature drift due to the analog sensing and outputting mechanism utilized. Pressure transducers are known to be inaccurate for position sensing applications (particularly in demanding downhole environments), and the installation of such sensors tends to be complicated and expensive, e.g., requiring o-rings and/or other seals. The above-described drawbacks of prior art sensor arrangements often result in unreliable and inaccurate position data and also tend to increase the fabrication and maintenance expense of downhole tools.
Therefore, there exists a need for an improved sensor apparatus and method for accurately determining a position and/or distance of various downhole tool components. In particular, there exists a need for improved downhole tool position sensor deployments, e.g., including wireline, logging-while-drilling (LWD), measurement-while-drilling (MWD), and steering tool deployments.